Vinylic CH Borylation of Cyclic Vinyl Ethers with Bis(pinacolato)diboron Catalyzed by an Iridium(I)‐dtbpy Complex

Abstract Borylation of the vinylic CH bond of 1,4‐dioxene, 2,3‐dihydrofuran, 3,4‐dihydro‐2H‐pyran and their γ‐substituted analogs was carried out in the presence of bis(pinacolato)diboron (B 2 pin 2 ) and a catalytic amount of Ir I ‐dtbpy (dtbpy=4,4′‐di‐ tert ‐butyl‐2,2′‐bipyridine) complex. The two boron atoms in B 2 pin 2 participated in the coupling, thus giving two equivalents of the coupling product from one equivalent of B 2 pin 2 . The borylation of 1,4‐dioxene in hexane resulted in 81 % yield at room temperature. The borylation of 2,3‐dihydrofurans at 80 °C in octane suffered from low regioselectivity, and gave a mixture of α‐ and β‐coupling products even for hindered γ‐disubstituted analogs, but γ‐substituted analogs of 3,4‐dihydro‐2H‐pyran achieved high α‐selectivity, giving single coupling products. This protocol was applied to the syntheses of a key precursor of vineomycinone B2 methyl ester and other C‐substituted D ‐glucals by borylation of protected D ‐glucals with B 2 pin 2 to give α‐boryl glucal followed by cross‐coupling with haloarenes, benzyl bromide, and allyl bromide. A catalytic cycle that involves the oxidative addition of sp 2 CH bond to iridium(III)‐trisboryl intermediate as the rate‐determining step has been proposed.